# Build on Bitcoin bridge

Any application or user on the target chain can utilize the bridge to verify the inclusion of a particular transaction on Bitcoin. For instance, a user could demonstrate to a smart contract that a specific amount of BTC has been transferred to a particular address on Bitcoin. Additionally, a user might showcase ownership of a unique Ordinal on Bitcoin to the contract. To initiate a query to the bridge and confirm the inclusion of a specific transaction ID on Bitcoin, the user must pay the bridge fee, which is collected in the native token of the target chain.

## Transaction inclusion query

The Bitcoin Relay has a function called `checkTxProof` that can be used to check the inclusion of a transaction in a block:

```solidity
function checkTxProof(
    bytes32 txid,
    uint blockHeight,
    bytes calldata intermediateNodes,
    uint index
) external payable returns (bool);
```

Where:

* `txid` is the double hash of a Bitcoin transaction.
* `blockHeight` is the height of the block that includes the transaction.
* `intermediateNodes` is the Merkle inclusion proof of `txid` in the block.
* `index` is the index of `txid` in the transactions of the block.


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